1. Field of the Invention
The present invention concerns a regeneration control method for continuously regenerating a diesel particulate filter device provided with a filter, which purifies the exhaust gas by collecting particulate matter of the diesel engine.
2. Detailed Description of the Prior Art
The exhaust gas quantity of particulate matter (PM: particulate matter: referred to as PM hereinafter) exhausted from the diesel engine is regulated strictly year by year with NOx, CO, HC and so on. Technologies for reducing the quantity of PM discharged outside, by collecting this PM with a filter called “diesel particulate filter” (DPF: Diesel Particulate Filter: referred to as DPF hereinafter) are developed.
The DPF for collecting PM includes a monolith honeycomb wall flow type filter made of ceramic, a fiber type fitter made of ceramic or metal fiber, and so on. The exhaust gas purifier using these DPFs is installed in the middle of the engine exhaust pipe, similarly to the other exhaust gas purifiers, for purifying exhaust gas generated by the engine.
However, in this DPF for collecting PM, clogging progresses along with the collection of PM and the exhaust gas pressure (discharge pressure) along with the increase of the quantity of collected PM, requiring to remove PM from this DPF; hence, several methods and apparatuses are being developed.
For these apparatuses, those of a system for installing two (2) circuits of exhaust passage, each provided with a DPF, and for collecting PM and burning the collected PM to regenerate the filter alternately, also those of the continuously regenerating system for forming an exhaust passage with a single circuit, and for performing the treatment operation for filter regeneration to oxidize and remove the collected PM, all the way collecting PM by a DPF installed in this exhaust passage, are proposed.
The apparatus of this continuously regenerating system, described above, includes a continuously regenerating type DPF device provided with an oxidation catalyst upstream the DPF, called CRT (Continuously regenerating trap), a continuously regenerating type DPF device for reducing the combustion temperature of PM by the effect of a catalyst carried by the filter, called CSF (Catalyzed Soot Filter) to oxidize and remove PM by the exhaust gas, and so on.
As shown in FIG. 10, the continuously regenerating type DPF device 20A called CRT, utilizes the fact that PM is oxidized by nitrogen dioxide at lower temperature rather than the case where PM is oxidized by oxygen (O2) in the exhaust gas G, and is composed of an oxidation catalyst 21A and a filter 22A. Nitrogen monoxide (NO) in the exhaust gas G is oxidized into nitrogen dioxide (NO2) by the upstream side oxidation catalyst 21A carrying platinum or the like, and PM collected by the downstream side filter 22A is oxidized, by this nitrogen dioxide (NO2), into carbon monoxide (CO2), for removing PM.
Also, as shown in FIG. 11, the continuously regenerating type DPF device 20B called CSF is composed of a filter 22B with catalysis which has a catalyst such as cerium oxide (CeO2). Moreover, in the area of the lower temperature (300° C. to 600° C.), mainly, PM is oxidized through a reaction (4CeO2+C→2Ce2O3+Co2, 2Ce2O3+O2→4CeO2 and so on). Besides, in the area of the higher temperature (not less than 600° C.), PM is oxidized by oxygen (O2) in the exhaust gas G.
However, for these continuously regenerating type DPF devices as well, in case of lower exhaust temperature or during an operation state with a low quantity of nitrogen monoxide (NO) emission, the aforementioned reaction for PM oxidation and removal does not occur due to the lowered catalyst activity followed by the lowered catalyst temperature, or lack of nitrogen monoxide (NO). Accordingly the filter cannot be regenerated and PM continues to build up in the filter, resulting in filter dogging.
Consequently, in these continuously regenerating type DPF devices, a regeneration control is performed for estimating the quantity of the collected PM in case of regenerating the filter, and for oxidizing and removing PM caught in the filter through a forced elevation of exhaust temperature or increase of the quantity of nitrogen monoxide (NO) emission, by changing the engine operation state to the regenerating mode operation, in case where this estimated quantity of the collected PM exceeds a predetermined value.
Then, in the conventional continuously regenerating type DPF device, a regeneration control judges when to start the regenerating mode operation, in case where the estimated quantity of the collected PM exceeds a predetermined judgment value by clogging the filter, the device performs the regeneration control to switch to the regenerating mode operation regardless of engine operation state at the time of the judgment.
In this regenerating mode operation, because it is necessary to heat an oxidation catalyst or filter with catalysis to a temperature not less than the predetermined temperature for activation of the catalyst of the oxidation catalyst or filter with catalysis, it is so composed to maintain the catalyst temperature not less than the activation temperature by heating it up with the exhaust gas.
For instance, if a regenerating mode operation including a post-injection is performed, as this post-injected fuel gets burned in a down stroke of the piston, the contribution to the engine output is low, while the contribution to heating up the exhaust is high, which is effective for heating up the exhaust.
However, in this post-injection, the whole of the injected fuel cannot complete the combustion entirely in a cylinder, but a part thereof is emitted into the exhaust passage as unburned HC or CO. At this moment, if the catalyst temperature becomes not less than the activation temperature, these unburned HC or CO is oxidized by the catalyst and contributes to heating up the exhaust. However in case where it has not become not less than the activation temperature, the unburned HC or CO will be emitted as they are without contributing to heating up the exhaust, thereby causing pollution. Moreover, the filter regeneration becomes insufficient.
On the other hand, since the engine operation states has various states, at the time when it is judged to start the regenerating mode operation, it is necessary to raise the exhaust gas temperature to not less than a fixed temperature, during the regenerating mode operation. So, when the exhaust temperature is low as in the case of low speed operation, low load operation, and so on, an operation to heat up the exhaust gas is performed to coercively raise the exhaust gas temperature.
For instance, in the idling operation, low speed operation or engine brake functioning operation on the downhill etc, the fuel is burned scarcely, and exhaust gas at lower temperature flows into the continuously regenerating type DPF device, thus the catalyst temperature and the catalyst activity will be lowered.
Especially, in case of using a vehicle having a continuously regeneration type DPF device on board is used for door-to-door delivery service or the like dominated by the urban area traveling, the engine is often operated with a low exhaust gas temperature, thus in the regenerating mode operation, it is often necessary to perform the control for heating up the exhaust to raise the exhaust gas temperature.
Then, in the conventional control method for heating up the exhaust gas, as only a single kind of control for heating up the exhaust gas composed by combining several of, preset, injection timing retard (delay) of fuel injection, post-injection, admission throttling, exhaust throttling, EGR, load increase by driving an auxiliary, heating up the exhaust gas by a heating means such as electric heater, burner or the like, and so on, is performed. Therefore the operation for heating up the exhaust gas turns up to be executed by this single kind of control for heating up the exhaust gas, independently on the catalyst temperature of that time, if the exhaust gas temperature, catalyst temperature or others are not more than a predetermined temperature.
However, since the control for heating up the exhaust gas provided only in this single kind, is composed to securely heat up an exhaust gas of the supposed lowest temperature, this control for heating up the exhaust gas performs the operation for raising temperature, which is far from the operation state of idling operation or low speed operation etc.
Consequently, this control for heating up the exhaust gas for coercively raising the exhaust gas temperature provokes the problem of deteriorated fuel consumption, because heat energy supplied from the fuel or the exterior is consumed unnecessarily for heating up the exhaust gas, or unnecessary equipment is driven. Further, there is a problem of deterioration of drivability, because engine output variation is provoked by this control for heating up the exhaust, when it is changed over to the regenerating mode operation during the driving.